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Path loss survey system

URS-1 was a complete test system for investigating the path attenuation or path loss in a specific 316 MHz covert radio system, developed in 1971 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA). It was used to analyse the link budget 1 of the communications system, of which a series of tailor-made Sleevex antennas formed an important part [2][3].

The complete URS-1 system was supplied in a common Samsonite briefcase, and consisted of an URR-1 receiver, two URT-1 transmitters, an LP antenna, batteries, attenuators, accessories and two sets of three Sleevex antennas each.

The image on the right shows a typical URS-1 set, as used by the CIA, in the original briefcase. The front panel of the URR-1 receiver is visible. The URR-1 is powered by two 9V batteries. In the foreground is an URT-1 transmitter, which is powered by a single 9V battery. A green Sleevex antenna is connected to the URT-1 transmitter.
Transmitter with Sleevex antenna

The Sleevex antennas and the URS-1 were developed by the NRP especially for communication projects of the CIA in the 315 MHz band, as part of a long-term research contract under the name Easy Chair. A Sleevex antenna is no thicker than 10 mm and is basically a vertical dipole that is fed by a coaxial cable via one of its conductors. This antenna type is known as a coaxial antenna or sleeve antenna. Specific versions were made for use in free space, concrete and wood. They were also made for other frequencies. The CIA used the URS-1 also for training purposes.

  1. In a telecommunications system, the link budget is the sum of all gains and losses from transmitter, through the medium to the receiver.  Wikipedia

Lifting the upper part of the case Contents of the Samsonite briefcase Accessories stored at the second level URT-1 transmitter and URR-1 receiver Transmitter with Sleevex antenna URT-1 transmitter 300 MHz URR-1 receiver
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Lifting the upper part of the case
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Contents of the Samsonite briefcase
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Accessories stored at the second level
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URT-1 transmitter and URR-1 receiver
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Transmitter with Sleevex antenna
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URT-1 transmitter 300 MHz
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URR-1 receiver

The diagram below shows the basic setup of the URS-1 system. At the left is the transmitter that is fed by a 9V battery which is installed at the bottom end. An external dynamic microphone can be connected to it, but it can also be replaced by a shorting link, in which case the transmitter produces a constant beep. The transmitter is normally connected to one of the Sleevex antennas.

At the right is the URR-1 receiver that should be connected to the SRN-9M reference antenna. The receiver produces a visible output on an indicator at its front panel, and an acoustic one through a suitable pair of headphones. An extra output is available for connection of a recorder.

The illustration below shows some of the factors that attribute to path loss, starting with the type and position of the bug and its antenna at the target area. The transmitter's signal is attenuated by the distance to the receiver, the material in which the antenna is embedded, the walls of the building, any furniture, and by anything else that is in the signal path to the Listening Post (LP).

Possible causes of path loss

Other factors may attribute to the gain of the signal, such as the transmitter's output power, the gain (if any) of its antenna, the gain of the receiving antenna and any pre-amplifiers. In order to predict the propagation of the signal with some degree of reliability, it may be useful to calculate the sum of all gains and losses and compare it to the link budget of the entire system. A detailed path loss survey was usually carried out by the CIA before planting a bug at a given target.

The diagram below shows how and where the various items are packed inside the Samsonite briefcase. The bottom section is removable and gives access to a further compartment that contains the accessories, cables, adapters, spare parts and a collection of Sleevex antennas.

Contents of the Samsonite briefcase

Modified Samsonite briefcase Universal Radio Transmitter Universal Radio Receiver Listening Post (LP) antenna Collection of Sleevex antennas Cable for connection to recording equipment Three 20dB attenuators Full technical manual
Samsonite briefcase
All parts and accessories of the URS-1 survey system were supplied in the standard Samsonite executive style briefcase shown in the image on the right. The documentation and the reflector of the receive antenna are stowed in the top lid.

All other parts are stowed in the bottom part, that consist of two layers. The transmitter and the receiver are directly accessible. Adapters and cables are stored in the bottom section.
Samsonite briefcase with URS-1

Transmitter   URT-1
The transmitter measures 160 x 27 x 19 mm, including the 9V battery that powers it. It works on a single crystal-operated channel (314.500 MHz in this case), with an optional tone or voice modulation. A microphone can be connected externally to the solder terminal on top of the device. The transmitter has a BNC socket for connection of a Sleevex antenna.

Unlike other bugs, such as the SRT-107, the URT-1 does not provide audio masking and will therefore be much easier to detect.

 More information

URT-1 transmitter 300 MHz

Receiver   URR-1
The URR-1 receiver was designed especially for path loss measurements and can best be seen as a calibrated field-strength meter for the 314-316 MHz frequency range. It should be used in combination with the SRN-9M receive antenna.

It has three crystal operated channels: (A) 314.5 MHz, (B) 315.5 MHz and (C) 316.5 MHz. The receiver is powered by two 9.5V batteries that are fitted in a small compartment at the rear.

 More information

URR-1 receiver

Receive antenna   SRN-9M
The SRN-9M was supplied as a reference antenna for the URR-1 receiver. It is basically a center-fed half-wave (½λ) open dipole that is placed before a reflective shield. The total gain of this antenna is estimated at 8.5dB.

Inside the boom (between the two dipole elements and the reflector) is a BALUN that provides correct matching to the coaxial transmission line.

 More information

Directional LP antenna

Sleevex antennas
The URS-1 set was supplied with two complete sets of Sleevex antennas (three models each), so allow simultanious testing with two independent transmitters in combination with the URR-1 reference receiver.

Each Sleevex antenna has a colour coded ring at its base, that indicates the environment (medium or diëlectricum) for which it has been designed.

 More information

Yellow Sleevex antenna

The URR-1 receiver delivers its audio to a pair of headphones that can be connected to the 6.3 mm jack socket at the front panel. In addition, the received signal can be recorded onto a (tape) recorder, via the 3-pin DIN socket at the front.

A suitable cable for connection to a recording device was provided with the set. It is shown in the image on the right.
Recording cable

In order to obtain a proper and calibrated result from the measurements, it may be necessary to attenuate the signal that is intercepted by the receiver. For this purpose, three calibrated 20dB attenuators are supplied with the set.

The image on the right shows a single 20dB attenuator, that can be inserted directly between the antenna and the RF input of the receiver. Note that the actual attenuation is slightly frequency dependent, as indicated on the label.
20dB Attenuator

The URS-1 was supplied with a complete technical manual, that contains the full circuit diagrams of the transmitter and the receiver, plus full circuit descriptions.

In addition, it provides detailed information about path loss measurements and antennas, and gives instructions and tips on how the measurements are best carried out in order to obtain the most reliable results. This manual does not contain any classified information.

 Download the manual


URR-1 receiver Front panel 20dB Attenuator Three 20dB attenuators
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URR-1 receiver
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Front panel
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20dB Attenuator
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Three 20dB attenuators

  • Battery 9V (4x)
  • Screwdriver
  • Various coaxial cables
URS   Universal Radio System
Complete system for carrying out reference measurements. Also used for training purposes.
URT   Universal Radio Transmitter
Simple transmitter for training perposes and for reference measurements. Supplied as part of a complete reference survey system.
URR   Universal Radio Receiver
Universal receiver for reference and field-strength measurements. To be used under varuing conditions, in order to obtain the best possible propagation of the signal.
  1. Technical Manual for URS-1
    February 1977.
  1. Anonymous, URS-1 path loss survey system
    July 2016.

  2. Wikipedia, Path loss
    Retrieved january 2017.

  3. Wikipedia, Link budget
    Retrieved january 2017.
Further information
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Crypto Museum. Created: Tuesday 03 January 2017. Last changed: Sunday, 13 August 2017 - 07:42 CET.
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